The great bulk of hardwood lumber goes to some wood working plant where it is made into furniture, flooring, cabinets, or other products. Next to sawing, planing is by far the most important machining operation. Nearly every hardwood board is planed at some stage of its fabrication into a finished product.
The tip of such a machine employs a cutterhead in which a plurality of cutter blades are spaced around the periphery of the cutterhead with each blade aligned with the axis of rotation of the cutterhead. A typical example is described in U.S. Pat. No. 2,702,059.
In such a machine the entire cutting edge of each successive blades simultaneously engages the surface of the wood. Thus, there is a tendency for the rotating blades to "hammer" the workpiece resulting in excessive noise. Moreover, the blade becomes heated quite quickly which accelerates its becoming dull.
U.S. Pat. No. 2,969,816 describes an improvement wherein the cutterhead blades are angularly disposed relative to the axis of rotation. Because of the angular disposition of the blades, the cutting edges of the blades are convex in shape. The noise of the machine is reduced and the blades tend to stay sharper longer; however, a convex edged blade is not easy to keep sharp.
U.S. Pat. No. 4,541,756 describes an even more recent improvement in the art. There a blade is used having an elliptically curved cutting edge. Some manufacturers have gone a step further and used a cutterhead having helical cutting edges (i.e., like on a hand driven reel lawn mower) in order to effect smooth and silent machining.
Unfortunately, it is very difficult and expensive to manufacturer cutting blades having other than a flat, smooth, straight, cutting edge. When helical cutting edges, elliptical cutting edges, or convex edged blades are used, the blade is not only expensive to manufacture but more difficult to sharpen at the job site. The sharpening technique is not easy and often special purpose equipment is needed. The net result is the benefits of axially inclined cutting blades are not easily or practically achieved.
Another factor affecting the use of the cutterhead of a planer is its cutting angle. The cutting angle is the angle between the face the blade or cutting edge and a radial line. The importance of cutting angles as a factor in the quality of planing varies greatly among the species of wood. The oaks, for example, are not much affected and plane well through a wide range of angles. On the other hand, Hackberry and Willow may yield three or four times as many defect-free samples when an optimal cutting angle is used. A wood working plant that specializes in one product, such as oak flooring, has only one wood to consider and can easily adapt its practices to the particularities of that wood. However, a general planing mill, a custom wood working plant, or even the individual having a woodshop in his home, handles a wider variety of wood species. Since it is not practical to change knife angles every few hours with a change of species, one cutting angle is usually adopted that experience has shown to give the best results for a given set of conditions. As a rule, a 20 degree cutting angle is good if the species are hardwoods and a 30 degree cutting angle if good if softwoods are the chief raw material. Often the wood worker purchases at least two sets of cutterheads with each cutterhead having blades inclined in a different cutting angle. However, this is an expensive solution to the problem and one not often practical to the small woodshop.
If standard, straight edged, flat cutting blades could be used, if the these blades could be inserted into a cutterhead without excessive need for alignment if such a cutterhead could be incorporated in existing planer machines, and if the cutting angle could at the same time be optimized, productivity and quality would be improved at relatively little cost. Moreover, existing planer machines and woodshops would be given added utility.
What is needed is a cutterhead for planing machine which achieves the benefits of low noise and high quality without the expense of having to buy special purpose or especially shaped blades or a plurality of fixed angle cutterheads.